Unlike broadly active chemotherapeutics which indescriminately kill dividing cells, select targeted cancer therapeuctics have demonstrated the potential to specifically eradicate cancer cells while sparing “normal” non-cancerous cells, resulting in clinical efficacy and minimized adverse side effects. Nevertheless, most single agent cancer therapies fail in the clinic due to underwhelming anti-tumor responses. Even in the few cases where targeted agent monotherapies have succeeded in treating solid tumors, the effect is usually transient and drug-resistant tumors quickly reemerge. One approach to improve clinical outcome of anti-cancer pharmaceuticals is the combination of two or more therapies, an approach that oncologists have utilized for decades with broadly active DNA-damaging agents. More recently the strategic pairing of targeted oncology agents has gained momentum with the hope of synergistic cytotoxicity, making the combination more effective in treating tumor cells than either single drug alone. Drug combinations are expected to take advantage of synthetic lethality or repressing compensatory feedback mechanisms that would otherwise allow cancer cells to survive effects of monotherapy. Optimal combinations might also delay onset of drug resistance by killing more tumor cells as well as by limiting alternate means of developed cellular resistance.
Small molecule inhibitors against checkpoint kinases constitute a promising class of targeted cancer therapeutics and many are currently under clinical evaluation. CHK1 is an essential serine/threonine kinase involved in two cell cycle checkpoints, the intra-S and G2/M checkpoints. In response to DNA replication stress during S-phase of the cell cycle, CHK1 activity prevents stalled replication forks from collapsing and causing genomic damage (Feijoo, C., et al., J. Cell Biol., 2001; 154(5):913-923). Also, CHK1 activity following DNA damage is necessary for arrest at the G2/M cell cycle boundary, preventing cells from prematurely entering mitosis before damaged DNA has been repaired (O'Connell, M. J., et al., Embo Journal 1997, 16(3):545-554; Liu, Q. H., et al., Genes & Devel., 2000, 14(12):1448-1459). Importantly, CHK1 is necessary for unperturbed DNA replication and cell cycle coordination even in the absence of any exogenous insult. As an example, conditional CHK1 heterozygosity leads to abberant DNA replication, increased DNA damage, and premature mitosis in untreated murine mammary epithelial cells (Lam, M. H., et al., Cancer Cell, 2004, 6(1):45-59). Several publications describe the cytotoxic nature of CHK1 knockdown or inhibition, either alone or in combination with DNA-damaging therapeutics, demonstrating preclinical proof of concept for CHK1 targeted agents.
WEE1 is an essential tyrosine kinase best recognized as a mitotic gatekeeper that phosphorylates and inactivates cyclin dependent kinase 1 (CDK1=CDC2), the only indispensible human cyclin dependent kinase (Malumbres, M. and Barbacid, M., Nature Reviews Cancer, 2009, 9(3):153-166). As cells transition into mitosis, WEE1 activity is reduced, allowing CDK1/cyclin B1 to intiate mitotic events. WEE1 is therefore critical for properly timing cell division in unperturbed cells, and loss of WEE1 results in chromosomal aneuploidy and accumulated DNA damage (Tominaga, Y., et al., Intl. J. Biol. Sci., 2006, 2(4):161-170). Additionally, WEE1 activity can be increased as a result of DNA damage, causing cells to arrest in G2 and allowing for repair of DNA lesions before beginning mitosis (Raleigh, J. M. and O'Connell, M. J., J. Cell Sci., 2000, 113(10):1727-1736). Recently, WEE1 has been shown to be indispensible for genomic integrity specifically as cells traverse S-phase, describing a previously unrecognized role for WEE1 in maintaining fidelity of DNA replication, (Beck, H., et al., J. Cell Biol., 2010, 188(5):629-638). Knockdown of WEE1 by siRNA led to rapid and S-phase specific accumulation of γH2AX, a phosphorylated histone protein that quantitatively represents DNA damage. Interfering with WEE1 has been shown to repress cancer cell proliferation and lead to greater anti-tumor effects of DNA-damaging chemotherapeutics than either single agent alone could achieve.